Tom Vanderbilt, the traffic guru, has authored a piece in Slate today that advocates getting rid of the left turn.

Forty percent of accidents happen at intersections, he said, most of them involving crossing into the path of traffic.

What’s a better solution. I give you — er, he gives you — the diverging diamond interchange:

As the eastbound driver approaches the highway interchange (whose lanes run north-south), traffic lanes “criss cross” at a traffic signal. The driver will now find himself on the “left” side of the road, where he can either make an unimpeded left turn onto the highway ramp, or cross over again to the right once he has gone under the highway overpass.

What makes the DDI work is that it reduces the number of “conflict points” where traffic streams cross each other. There would usually be 26 such points in an intersection like this, but the DDI has only 14 (because, for example, drivers turning onto ramps no longer have to turn across oncoming traffic). But… not having those left-turn movements adds another advantage. In a standard “diamond” interchange, where traffic entering the highway has to turn across traffic, the two sets of traffic signals, because they have to account for the left-turn phase, are difficult to synchronize–which means cars wait in longer queues. But with the DDI, Chlewicki told me, “each signal in the interchange is only two phases, not three. And each of these two phases have some unique characteristics. The left turn from either ramp gets the same green phase as the arterial thru movement that does not conflict with that turn.

Guess what state is — literally — heading in this direction? Minnesota. Specifically Pine Island. The Highway 52 interchange there is a particular nightmare.

Vanderbilt points out, however, that the underlying problem in any traffic design is congestion. As long as we keep building houses with three-car garages, he says, traffic problems are likely to get worse (and more dangerous) no matter what the design.

Vanderbilt will discuss his idea during a segment of today’s Talk of the Nation from NPR.

About the blogger

Bob Collins has been with Minnesota Public Radio since 1992, emigrating to Minnesota from Massachusetts where he was vice president of programming for Berkshire Broadcasting Company. Previously, he was an editor at the RKO Radio network in New York, and WHDH Radio in Boston. He is the founder of the MPR News’ website.

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Interesting idea. Here in Princeton, we recently got a roundabout at an intersection with a lot of left-turn crashes. I love the smoothness, but it is a confusing change at first.

What are the benefits of this idea over the roundabout?

Jim Shapiro

Traffic circles also eliminate left turns.

JB

Bloomington is reconstructing the I-494/34th Ave interchange as a diverging diamond as well. I don’t think it’s scheduled until at least 2014 though.

http://www.greatdiv.com Charlie Quimby

Roundabouts are common in my Colorado hometown, though still controversial among those who think it’s some kind of social engineering because the provide traffic calming as well as safety.

Looks like this is designed for roads with higher speeds.

Many years ago, a friend wrecked my car & I didn’t have the money to fix it. I learned then you can go anywhere making only right turns, and I still apply that insight today.

LG

I’m from Michigan and there are many intersections, especially in the suburbs with median-divided roads, where there are no left in turns. Instead, there is the “Michigan left” — driving through an intersection to a U-turn spot. Turn when it’s clear and move to the right lane. Make the turn like you would any other right. (http://en.wikipedia.org/wiki/Michigan_left)

At spots in Mich. where that’s not possible, a lot of burbs are going to roundabouts, have apparently worked well.

Don

There is a diverging diamond interchange in Springfield Missouri (Missouri Highway 13 and Interstate 44) that I’ve used several times over the last few years. It defiantly takes some getting used to, but the traffic flows through nicely.

As for roundabouts, they’re not useful in this situation, a road crossing over or under a divided controlled access highway.

jon

Just get self driving cars already…

Civil engineers have had 100 years to fix the streets to make driving easier/safer etc. And we still end up with signs like “rough road” and “bump” (not just during construction, but permanent signs)

Mean while Mechanical engineers have been making cars safer since the advent of the seat belt.

For some reason, people will not accept self-driving cars unless they are absolutely, positively guaranteed to not be in an accident.

Never mind the fact that as they get into a car, there is no such guarantee.

Product liability as it is, no company will ever be able to provide a self-driving car unless indemnified by the government in case of accidents. Just like nuclear power. And the cost of an car accident is many orders of magnitude less than that of a power plant’s “whoops” moment.

I’m all for the self-driving car. I’d love that hour+ back in my day, that I could use to read the newspaper or something similar.

Hayley

I’m very intrigued by this! I was in a terrible car crash a few years ago because a car took a left turn in front of me. And even though I was not taking the left turn in that situation, I began avoiding them so I didn’t do that to someone else. I also don’t understand why there are walk signals while a car is allowed to take a left turn on a green. There are just too many things involved and someone is bound to not coordinate it correctly.

I think I prefer the SPUI. I was the Paul who asked the question about it vs. the Diverging Diamond that Mr. Vanderbilt punted on…

http://www.farces.com/ Michael Fraase

I like roundabouts, although the only ones I’ve used locally are the ones in Edina off France. I live on Grand Avenue in Saint Paul and I really wish every intersection had them.

I especially like how traffic flow is handled on El Camino Real in Silicon Valley: the left-turn lane (with arrow) is also used for U-turns, so if you miss your intersection it’s no problem just make a U-turn at the next one.

Both roundabouts and left/U-turn lanes are useful only on city/suburban surface streets where diverging diamonds are useful for interstates and other highways.

But then, I thought we’d have personal rapid transit by now.

V

As a native Michigander, I’ll second LG on the Michigan left. But it doesn’t work very well if you’re new to the area, even if the intersection is marked as no left turn. Some people just have to see the turn coming.

http://www.mngun.com Andrew

Three-car garages don’t cause congestion. It’s all that lousy freedom that lets people in this land of plenty live in their own houses and not packed in like sardines to be convenient to cattle cars.

Tristan

I’m kind of intrigued by this idea. Just the other day I was attempting to make a left hand turn on a solid green. The car opposing me was a large SUV and there was no way to see around it to make sure that it was clear. Finally, I just floored it and took off and somebody almost smoked my right rear quarter panel.

Generally, I’ll do “the dance” with another left turner where if I notice they’re trying to see around me, I have a small car, I’ll get as far into the turn lane and turning as I can and hope they reciprocate.

The problem I have with rotary intersections is that most people don’t know how to use them and stop for a lot longer than necessary even when there aren’t cars to which to yield.

The diverging diamond looks like it could really work if executed correctly. It would definitely take getting used to, but it could be fun.

Troubleshooter

The DDI does not eliminate the left turn. It removes the oncoming traffic from being in the way of the left turn.

As long as electromagnetic pulse is a possibility, there will be no self driving cars.

The real problem is the standard intersection, with cars crossing at 90 degree angles. But to fix it would require a change in how we divide up real property.